Superabsorbant Hydrogels: A Study of the Most Effective Application of Cross-linked Polyacrylamide Polymers

West Texas landscape.

I live in San Angelo, Texas, where drought has left its mark. The U.S Drought Monitor classifies San Angelo and the surrounding region as being in a severe to extreme drought. Farmers are struggling, money has been lost, livestock are suffering, and the landscape shows the all-too-familiar brown grass and cracked earth. In my West Texas town, between the extremely sparse rainfall and daily water restrictions, water should be treated like liquid gold. We should be doing everything we can to conserve what water we have.

If only there were a way to capture that much-needed moisture and lock it away in the soil, to be used when the rains cease and the soil begins to dry out. What if that possible solution came in the form of a white powder?

Background

Hydrogels before added water (left). Hydrogels after an added 48 ounces of water (right).

I was first introduced to this “white powder” when I organized our county’s first National 4-H Youth Science Day. This “white powder” was made up of hydrogels. During Youth Science Day, I taught elementary school kids about these “superabsorbent crystals,” which are commonly found in diapers and contact lenses. We did some fun basic experiments that involved adding water to small amounts of the white powder and watching it magically expand and develop a Jell-O-like consistency.

Acrylamide and Potassium Acrylate (left). Hydrogel Network (right).

Hydrogels are cross-linked polyacrylamide polymers (PAM). They are made up of water-insoluble acrylamide and potassium acrylate. Polymers are long parallel chains of molecules, and when cross-linked they create a network of polymeric chains. Water is brought into the network through the process of osmosis and quickly journeys into the central part of the polymer network, where it is reserved. This is when the hydrogels act as absorbing agents and take on the outward appearance of a gel. Amazingly, hydrogels can absorb up to 500 times their weight in water, and when their surroundings begin to dry out, the hydrogels, or cross-linked polyacrylamide polymers (PAM), gradually dispense up to 95% of their stored water. When they are exposed to water again, they will rehydrate and repeat the process of storing water. This process can last up to seven years, when the biodegradable hydrogels decompose. These properties are what make hydrogels attractive to the agricultural world.

So could I use this relatively new development on the forefront of science to do my part in conserving water? Could I apply hydrogels to my backyard and conserve water by retaining moisture in the soil?

Purpose Of Research

Although many nurseries have shown an interest in hydrogels, not as many farmers or homeowners have. I wanted to see if hydrogels would make a significant difference in conserving water and retaining the moisture in the soil in my backyard. Then perhaps this white powder could contribute to the solution of helping to conserve water in a time of drought.

I knew cross-linked polyacrylamide polymers absorbed water; I’d seen it with my own eyes. But I needed to know how much water they would conserve, and how much moisture the soil would retain. I also needed to know what application to the soil would be the most effective in retaining moisture and conserving water.

I divided my hydrogel applications into four zones:

a one-inch-thick application at the surface of the soil,

a one-inch-thick application in the root zone,

a one-inch-thick application below the root zone, and

spread throughout the soil from the surface to one inch below the root zone.

Application zones.

I believed that if the hydrogels were placed near the root zone, then the plant would be able to use the hydrogels as a water source when the soil became dry because the hydrogels would release their absorbed water as the soil around them became dry. This would make stored water available to the roots of the plants, thus decreasing the need for water and increasing water conservation.

Methods And Materials

One of the major goals of my project was to “represent the home front”—to observe and experiment with how hydrogels might work for ordinary users. Would hydrogels make a significant difference in a homeowner’s water bill?

I extracted enough soil from my backyard to use in my experiment, and use the U.S.D.A.’s Textural Soil Classification Chart to identify my soil type as silt loam. I insured a uniform blend of hydrogels throughout the soil by mixing them together thoroughly for my samples. I chose two plants for each container—an Ophiopogon japonicus and a Viola x wittrockiana , whose common names are mondo grass and pansy, respectively. These two plants were chosen because I’ve had them in my own yard, and they are quite common in our area. The Ophiopogon japonicus is a drought-tolerant plant, while the Viola x wittrockiana does not handle drought conditions well and needs to be watered regularly. I thought this would be a good “test” for the hydrogels to see where they excelled.

I chose six 11-by-6-inch plastic containers and drilled eight half-inch holes in the bottom of each container to simulate the drainage that would occur in a real garden. My controls and variables were set up as follows:

Drainage holes to mimic nature.

Control A: No hydrogels—watered once at beginning of experiment; Soil moisture retention control.

Control B: No hydrogels—watered regularly throughout experiment; Water conservation control.

Variable A: A one-inch-thick application of hydrogels at the surface of the soil

Variable B: A one-inch-thick application of hydrogels in the root zone

Variable D: An application of hydrogels spread throughout the soil, from the surface to one inch below the root zone.

Box used to measure soil sample.

The application of the hydrogels was very pertinent to the success of this project. The ratio of polyacrylamide polymers to each soil sample involved creating an instrument that measured a one-square-foot, one-inch-deep soil sample, and I used it to create soil samples of the same size for each of my variables.

Next I added one teaspoon of hydrogels to the measured sample, and using gloves, vigorously mixed the hydrogels into the dirt for three minutes. This helped ensure a uniform soil/hydrogel mixture. I filled my six containers according to their labels and planted one Ophiopogon japonicus and one Viola x wittrockiana in each of the containers, four inches from each other and two inches from either side.

Next, the process of watering the six different containers began. I slowly watered each container with exactly the same amount of water—40 ounces. The plants and soil had their first and only drink

Adding hydrogels to soil (left). Adding plant samples (right).

of water (except for Control B), and the drought began.

Control A represented soil moisture retention. It was watered once. This control was used to identify the day when the percentage of soil moisture was no longer capable of supporting plant growth—the day it hit the wilting point. Control B served as a water conservation control. It was watered regularly and was used to identify the amount of water needed in normal conditions to sustain the plants at a proper percentage of soil moisture. Every day I used the Soil Master Moisture Meter to measure the levels of moisture in each sample three times,

Megan uses a moisture meter to measure soil moisture (left). Plants at the beginning of the project (right).

and then took an average to ensure more accurate results. This was done every day for 30 days. Each day I took observations of the appearance of the plants.The Soil Master Moisture Meter was designed (according to a company representative) to measure the “degree of moisture” in the soil. With such an empirical measure, I wasn’t satisfied with just using degrees of moisture as my units. So I decided to calibrate my moisture reader. I used the gravimetric method to bring my soil sample to absolute 0% water moisture. I baked 600 grams of soil at 105ºC for 12 hours. I then added grams of water in small increments to a 300-gram sample of my 0% moisture soil sample. Using the following equation (% of water in soil = total weight of water added / weight of dry soil sample), I found the percentage of soil moisture at coordinating “degrees of moisture” on the moisture meter. This gave me the percentage of water in a given volume of soil.

Calculation of Percentage of Moisture in Soil by Grams, Starting with Absolute 0% of Moisture. Moisture Reading Data According to Moisture Meter.

Moisture Reading Data According to Moisture Meter.

Results

Immediately, the difference between the soil samples with hydrogels and the soil samples without hydrogels became evident. The dynamic rate of the water slowed down as it came in contact with the hydrogels. Already it was obvious that the cross-linked polyacrylamide polymers were at work absorbing the available water.

Controls A (left) and B (right).

Throughout the whole process, the differences between Control A and the variables were highly evident. Our soil is classified as a silt loam soil, and its capacity to hold water is low. Water passes through all zones quickly and away from the area available to plants. So seeing the water spread through Control A demonstrated the quick absorption rate of our soil. Throughout the 30 days, Control A decreased in moisture rapidly, which affected plant health.

Control B showed almost exactly the same results as Control A in absorbing water and retaining soil moisture at the beginning of the experiment. Of course, the difference was that Control B was watered as needed at the experiment proceeded. Control B showed the amount of water needed to sustain a proper percentage of soil moisture for the plants. Control B needed an additional 32 ounces of water during the experiment to sustain the proper percentage of soil moisture needed by the plants. As water was slowly added to the soil samples, the hydrogels again absorbed water into their “network” to store water for future use by the plants.

Variables A, B, C, and D (clockwise from left).

Interesting enough, Variable A, which contained hydrogels throughout the soil sample, showed the least amount of soil moisture, similar to Variable D. One explanation is that the hydrogels near the surface dried out sooner because of evaporation. Our humidity averaged 20 percent during my experiment. And since water is dynamic and not static, the water could have continued to move to the surface and evaporate, thereby lowering the moisture readings. Variable A retained optimal soil moisture for an additional five days and conserved 16 ounces of water during this experiment.

Variable B, which I hypothesized would be the best application, did indeed retain soil moisture and conserved water very effectively. It wasn’t until eight days after Control A dipped into the wilting zone that Variable B entered it. Throughout the period of the experiment, the plants thrived, and moisture levels were within an adequate moisture reading until the last two days. Variable B conserved 32 ounces of water during this experiment.

Variable C was very effective as it ran its course. Its moisture levels were very close to those of Variable B throughout the 30-day testing period. The difference between the two was very slight. Variable C also retained soil moisture above the wilting point eight days longer than Control A, eliminating the need to water during the testing period. Variable C conserved 32 ounces of water during this experiment.

Variable D declined more quickly than the rest of the variables. I believe this had to do with the rate of evaporation rate at the soil surface. This evaporation did not allow the stored water in the hydrogels to have a chance to be absorbed by the plants. With the water being so close to the surface, it quickly evaporated into the air.

I believe the results show a definite positive result in moisture retention in all the hydrogel applications, as well as a significant conservation of water. The most effective application of hydrogels was when they were applied in and below the root zone.

Conclusion

The future of cross-linked polyacrylamide polymers excites me. I was able to see water conservation happen first-hand. The soil samples containing hydrogels showed a large margin of water conservation, as well as better retention of moisture compared to the samples that did not contain hydrogels. Seeing the application of hydrogels work so well on such a small scale makes me excited to see their impact on a larger scale. I have my sights on my backyard.

My goal is to test cross-linked polyacrylamide polymers on my lawn and see what difference they will make. Over the years researchers have been improving hydrogels, and I believe that these superabsorbent crystals will only continue to improve. Already I can see that they could make a significant difference in the conservation of water at our house. I believe in being a good steward of our natural resources, and I want to do my part to sustain our water supply for future generations.

I have learned a lot about this “white powder” and the phenomenal potential it holds, and I desire to learn more. I would like to test the cross-linked polyacrylamide polymers’ performance in other soil types. Could they be used to create an oasis in the desert? I would also like to see how they react with different types of liquids. We all know that it is not always water that gets poured into our soil. How would these superabsorbent gels react to fertilizers, pesticides, gasoline, or other toxins? Would it create a concentration of toxins that could get into our food?

Although I still have many questions about these polymers, I believe that superabsorbent cross-linked polyacrylamide polymers have a very absorbing future! And I am thrilled to do my part in joining the water conservation effort to quench the thirst of the arid Earth.

Bibliography

American Soil Technologies, Inc. “Frequently Asked Questions.” Retrieved from the World Wide Web on 8 October 2008. www.americansoiltech.com.

Begnaud, John. Tom Green County extension agent for horticulture. Interviewed by e-mail. San Angelo, Texas. 19 September 2008 and 22 October 2008.

“Textural Soil Classification.” Natural Resources Conservation Service, United States Department of Agriculture. Retrieved from the World Wide Web on 19 February 2009. http//soils.usda.gov/technical/aids/investigations/texture

“Water and Sanitation.” Water Partners International, Fact Sheet No. 112, November 1996. Retrieved from the World Wide Web on 19 February 2009. www.water.org/FileUploads/FAQ_WHO_factsheet112.PDF

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Teacher Tip

Supplement a study of Earth science with an activity drawn from this winning student essay.

Ask students what diapers and contact lenses have in common. Have any of them ever heard of hydrogels?

Send students to this online article, or print copies of the essay for them to read.

Have them write a one-page reaction to the essay, focusing on what they learned about cross-linked polyacrylamide polymers (PAM) and their properties.